The present invention relates to satellite communication systems for exchanging data of different bandwidths, data-rates and delay requirements, between terrestrial terminals (user terminals, gateways, etc.) and in particular to such systems that utilise at least one geostationary satellite with a range of beams typically between 5 and 70. The number and size of the beams in the uplink and downlink directions may be different.
An architecture that is currently adopted for satellite communication systems to offer broadband and multimedia services to supplement and enhance service offerings is based on bent pipe geostationary satellite networks. An example of such a system uses Ka-band on the return path (from the user terminals to the gateways) and Ku-band Digital Video Broadcast (DVB) on the forward or outward path (from the gateways to the user terminals). The open and neutral nature of this architecture provides bandwidth-on-demand and broadband access that are both attractive and ubiquitous when compared to terrestrial solutions. However, the current architecture has certain limitations.
Firstly, only one hub or a handful of remote gateways are used to interconnect user terminals to terrestrial networks. This works well for local traffic but requires terrestrial back-haul of traffic when traffic becomes global. The necessity to use terrestrial networks for global traffic significantly increases the costs of operations and makes a satellite solution less attractive for Telecomms operators.
Secondly, this architecture does not allow for direct user terminal to user terminal connectivity, through one satellite hop, where the user terminals are located in different beams of a satellite, known as mesh connectivity,. This limits the use of the satellite communication system because user terminal to user terminal connectivity becomes costly in terms of satellite resource usage (since two hops are required) and can only be used for applications which are not time delay sensitive.
The advancing satellite on-board processing technologies have the long-term objective of providing full connectivity at a packet granularity by the use of on board packet switching and cell relay. However, the high technological risk of this alternative is not compatible with the time scales envisaged for providing the next generation of broadband satellite communication systems. Hence there is a need for intermediate solutions that trade-off features for risks.
An architecture for a satellite communication system to solve the first problem discussed above has been introduced. An on-board processor is used to implement carrier based routing on the return link. In this way a given group of contiguous carriers on the return up-link in a satellite beam number i can be switched to a required gateway down-link j, without demodulation. This provides a drastic simplification of the payload required for return path routing as compared to an onboard switch at a packet granularity. The forward or outward link comprises a continuous TDM stream made of time frames within superframes which are preferably DVB based. The forward up-link TDM streams coming from a gateway on uplink beam k is demodulated, and depending on the air interface, is at least partially decoded, by the satellite payload. An on-board TDM router with the capability of switching frame by frame to the desired satellite downlink beam l according to each position within the superframe. This architecture again allows a simplification of the payload with respect to a switch payload operating at packet granularity, particularly since no buffering is involved.
Thus, the granularity of switching on the return link is a carrier or group of carriers and on the forward link is a frame. It should be noted that the forward link is regenerative and the return link is transparent, ie. non-regenerative.
This approach allows for least cost routing (ie. a direct connection between a user beam i and a gateway beam j irrespective of its location) and significantly reduces the dependence of the system on terrestrial networks for transport of non-local traffic, while minimising the risks associated with on-board processing. However, this system architecture does not provide a mesh connectivity capability in a better way than the initial bent pipe system, as user terminal to user terminal connections require two hops of the satellite. This is not compatible with some time delay sensitive services, such as video conferencing, which customers may require.
The present invention aims to improve upon existing satellite communication systems by improving user terminal to user terminal connectivity, so called mesh connectivity, whilst keeping the technological risk at a manageable level.
According to a first aspect of the present invention there is provided a satellite communication system having a spatial segment comprising at least one satellite carrying a multi-beam telecommunication payload and a terrestrial segment comprising access gateways to terrestrial networks each gateway arranged to communicate with a/the satellite via a forward uplink beam and a return downlink beam and user terminals each arranged to communicate with a/the satellite via a forward downlink beam and a return uplink beam, wherein the forward link carries signals multiplexed in time and the return link carries signals multiplexed in frequency and time, the payload comprising;
a forward link router for routing signals multiplexed in time from a forward link uplink beam on a frame basis to a forward link downlink beam,
a return link router for routing signals multiplexed in frequency and time from a return link uplink beam on a frequency bank basis to a return link downlink beam, and
a mesh communication router for receiving signals multiplexed in frequency and time from a return link uplink beam, generating signals multiplexed in time and routing these signals to a forward link downlink beam.
The system according to the present invention allows a community of user terminals included within a same satellite beam to receive a single time division multiplexed stream of data on a forward link downlink which transports data from communications with a mix of gateways and other user terminals. Thus, the system can provide least cost routing and flexible beam connectivity allowing user terminals to communicate directly via one hop of a satellite. The user terminals can be relatively simple and low cost terminals capable of receiving only one receiver chain.
The system according to the present invention is particularly useful for a geostationary satellite as the time delay associated with a double satellite hop can be significant.
In a first preferred embodiment the return link uplink signals are received by the return link router and the return link router routes signals relating to a mesh communication to the mesh processor. This enables dynamic allocation of resource on the return link between access signals destined for gateways and mesh signals destined for other user terminals.
In a second preferred embodiment which is simpler than the first a proportion of the return link uplink signals are received by the return link router and a proportion of the return link uplink signals are received by the mesh processor. However, this reduces the flexibility of the system as the resource on the return link for access and mesh signals is allocated statically.
Each user terminal is allocated a home gateway to which the user terminal must make initial requests for resource on the system and in a preferred embodiment the system additionally comprises a bandwidth on demand controller (BoD) located in each home gateway. Alternatively, the bandwidth on demand controller can be located within the payload of the satellite.
The return link router is preferably transparent. This allows for the use of user terminals which will be compatible with next generations of satellite-based networks providing higher flexibility and connectivity characteristics. On the forward link, the compatibility of data streams with existing and widely used standards, such as DVB, ensures a backward compatibility with existing operator equipment and allows for a minimisation of user terminal receiver cost. The forward link router is preferably regenerative.
The forward link downlinks to the user terminals are preferably operated in broadband TDM, shared by all user terminals located within the same beam and the return link uplinks are preferably multiple access F/TDMA links.
The forward link router may be arranged to route successive frames independently in accordance with an interconnection matrix complying with a table stored in memory and computed by a network control centre which allocates the frames on the forward link uplink beams at each gateway. Similarly, the return link router can be arranged to route carriers independently in accordance with an interconnection matrix complying with a table stored in memory and computed by a network control centre which allocates the carriers on the return link to each gateway.
The present invention can be used in isolation or in combination with complementary payloads on the same satellite or complementary payloads carried by another satellite co-located in the same orbital slot.
According to a second aspect of the present invention there is provided a satellite which may be used in a communication system having a terrestrial segment comprising access gateways to terrestrial networks and user terminals, wherein the satellite has a multi-beam telecommunication payload comprising;
a forward link router for routing signals multiplexed in time from a forward link uplink beam from the gateways on a frame basis to a forward link downlink beam to the user terminals,
a return link router for routing signals multiplexed in frequency and time from a return link uplink beam from the user terminals on a frequency bank basis to a return link downlink beam to the gateways, and
a mesh communication router for receiving signals multiplexed in frequency and time from a return link uplink beam from the user terminals, generating signals multiplexed in time and routing these signals to a forward link downlink beam to the user terminals.
The satellite according to the second aspect of the present invention has the same advantages and the same preferred and alternative embodiments as are discussed above in relation to the satellite communication system according to the first aspect of the present invention.
According to a third aspect of the present invention there is provided a method of satellite communication for a satellite communication system; comprising the steps of;
at gateways to terrestrial networks, generating signals, multiplexing them in time and transmitting these signals to the satellite on a forward link uplink beam,
on-board the satellite, routing the signals multiplexed in time from a forward link uplink beam on a frame basis to a forward link downlink beam to user terminals,
at user terminals, generating access signals bound for a gateway and mesh signals bound for another user terminal, multiplexing the access and mesh signals in frequency and time so that the access signals are carried by access carriers and mesh signals are carried by mesh carriers and transmitting the access and mesh signals to the satellite on a return link uplink beam,
on board the satellite, routing access carriers from a return link uplink beam on a frequency bank basis to a return link downlink beam to the gateways and processing mesh signals from a return link uplink beam to generate signals multiplexed in time and routing these signals multiplexed in time to a forward link downlink beam.
The method may comprise the steps of dynamically allocating mesh resource and access resource on the return link uplink and receiving all the carriers on the return link uplink in a return link router on-board the satellite for routing access carriers to a return link downlink and routing mesh carriers to a mesh processor for processing mesh. Alternatively, the mesh resource and the access resource can be statically allocated and the method can comprise the steps of receiving the access carriers in a return link router on-board the satellite for routing access carriers to a return link downlink and receiving the mesh carriers in a mesh processor for processing the mesh.